The crystal structure of an aluminum-rich schorl overgrown by boron-rich olenite from Koralpe, Styria, Austria

Abstract

The first natural tourmaline (because tourmaline with [4]B has also been synthesized, we distinguish here between natural and synthetic tourmaline) that has been unequivocally demonstrated to contain B as a substituent at the T sites was described from Koralpe, Styria, Austria. This colourless B-rich olenite occurs as rims overgrowing schorl (black crystals up to a few cm) that has not yet been structurally characterized. A crystal structure refinement (R = 0.019) of this Al-rich schorl shows that [4]B occurs in the overgrown schorl; the optimized occupants of the atomic positions yield X (Na0.64Ca0.10K0.06□0.20) Y (Fe2+ 1.72Al1.08Ti0.11Zn0.03□0.06) Z (Al5.70Mg0.20Fe0.08 2+Mn0.02) ([3]BO3)3.00 T (Si5.76 [4]B0.24)O18 [F0.11(OH)3.31O0.58]. This is the first known (Al-rich) schorl where a structure refinement has detected [4]B. Comparing the structure refinements and the chemical composition of the Koralpe schorl and other [4]B-bearing tourmalines with tourmalines which contain no [4]B, it is of interest that only structure refinements of tourmalines which are low in magnesium and with a higher component of olenite show substantial amounts of [4]B; the role of Mg in controlling the amount of [4]B is not known, but it seems that an Al-component on the Y site (olenite-component), a boron-enriched environment and special P-T-t conditions are necessary to get tourmaline with substantial amounts of [4]B.